Why would a flourishing civilization, advanced for its time, suddenly cease to exist, its inhabitants gone and its architecture abandoned? Conspiracy theorists offer all manner of offbeat explanations including alien abduction, but in the case of these 12 societies, the causes were likely more mundane: natural disasters, climate change, invasions and economic irrelevance. Still, we don’t know – and likely never will – exactly what happened to bring about the end of the Khmer Empire of Cambodia, the Minoan society of Crete or two ancient civilizations right here in the United States.